Infrared radiation is typically characterized as the portion of the electromagnetic spectrum from 300 GHz to 400 THz. Many instruments and systems utilize an infrared (IR) light source. These include, for example, FTIR (Fourier Transform Infrared) spectroscopy systems and IR (Infrared) absorption systems. Applications for these instruments and systems include, for example, detecting trace quantities of hazardous substances, compositional measurements of various substances, obtaining information that leads to descriptions of properties (e.g., bonding) in various molecules, long-range measurements (including both integrated line measurements and point measurements at a distance) of potentially hazardous substances (e.g., biohazards, chemical hazards, pollutants, and irritants) in the atmosphere, and many applications in bioanalytical instrumentation, including FTIR microscopy.
Other applications for these instruments and systems also exist. For example, IR sources are used to calibrate instruments. For simpler temperature-measuring equipment, a conventional black-body source is used. However, for imaging IR systems, there is interest in having an array of IR emitters, where each emitter can be controlled independently of the others. Array's of IR emitters are typically implemented with an array of resistively-heated emitters. The temperature generated by resistively-heated emitters is typically limited to around 500 degrees Celsius. Resistively-heated emitters also have reliability issues and relatively slow time responses to achieve a desired temperature. Some users desire higher temperature capability than these emitters can provide—e.g., 1000 C or higher.
In cases where broadband IR light sources area needed (such as FTIR), heated elements are typically used. Numerous companies design and sell such elements, either by themselves or embedded into a larger piece of analytical instrumentation. Various materials are used for the heated elements, including tungsten, molybdenum, and silicon carbide. The materials used are typically electrically conductive and electricity is used to directly heat the element. While simple, inexpensive and compact, electrical heating of the elements limits the capability of the IR source. Furthermore, while heated-element IR sources are sometimes described in commercial literature as being capable of operating at temperatures up to approximately 1800 C, in fact, in real-world operation on analytical instrumentation, the temperature usually does not exceed about 1100-1300 C.
Several factors limit the maximum temperature at which conventional broadband IR sources may be operated. These factors include, for example, mechanical constraints (e.g., fatigue and stress where the electrical connections are made to the hot filament), heat loss through the electrical conductors, degradation of the source over time, and/or evaporation of the source over time. In some cases, another disadvantage of conventional IR sources is that as the source temperature is increased, the peak in the blackbody distribution shifts to shorter wavelengths, which is not where the radiated emission is most needed.